Bicycles have been utilized as a means of transportation for over a hundred years and continue to grow in popularity. The mountain bike is one type of bike that has become more and more popular in recent years. These types of bikes are often used in rough terrain, leading to an uncomfortable ride. Therefore, some recently designed bicycle frames include a rear suspension system. Bicycles equipped with suspension attempt to provide increased rider comfort, enhanced wheel contact and control, and less net rolling resistance without significantly increasing the cost, complexity or maintenance requirements of the bicycle.
However, no prior art suspension system meets all of these goals. Some systems are heavy, complex or require frequent maintenance. Some systems also cause adverse effects on other parts of the bicycle. For example, the distance between the crank axis, the axis that the pedals rotate around, and the axle of the rear wheel may vary with compression of the suspension system, forcing the transmission components to deal with increasing and decreasing amounts of chain slack. Some systems also create undesired reactions. For example, a rear suspension may cause “pogoing”, or movement of the axle through the rear wheel up or down relative to the rest of the frame as a reaction to pedaling forces. A rear suspension may also cause “biopacing” whereby movement of the crank arms accelerate or decelerate as a reaction to compression or decompression of the suspension. Pogoing and biopacing effects are particularly present in suspension systems where the chain slack varies as the suspension system compresses.
In particular, some rear suspension systems have multiple pivot points. These systems are generally complex and therefore expensive to manufacture. They also are subject to wear in multiple locations. U.S. Pat. No. 4,789,174, for example, describes a system having many pivot points including two pivot points directly above and below the rear wheel axle. These pivot points add significant complexity to the system and cause the distance between crank axis and rear axle to vary as the suspension system compresses.
U.S. Design Pat. No. 368,880 discloses another rear suspension system. In this system, a chain stay assembly pivots on the opposed ends of a bottom bracket tube. A pair of spring units connect the chain stays to the seat tube in about the location where the seat stays would normally be. The distance between the crank axis and rear axle remains constant, but this design requires two long stroke spring units, thereby increasing cost, complexity and maintenance.
U.S. Pat. No. 5,685,553 discloses another suspension system having a rear triangle that pivots on a frame. A bottom bracket and a rear wheel are both contained in the rear triangle. In this way, a fixed chain length is provided. However, pivoting of the rear triangle also moves the bottom bracket relative to the frame, causing the distance and angle between the seat and pedals to change while riding.